If you are being compensated for your services as an officer of the union, then it is very definitely considered self-employment income.
However, if you are only being reimbursed for out-of-pocket expenses for things that you do in connection with service to the union, then the treasurer is making a mistake by issuing any kind of 1099. As long as they have a written policy about reimbursing expenses and they require you to prove your expenses with documentation, they can reimburse the expenses without it being considered taxable income to you and without issuing a 1099. If this is for a 2020 tax return, you should discuss the situation with the treasurer before any 1099 is issued.
If you are being reimbursed for expenses but the union does not have a written policy about reimbursements or you do not provide proof of your expenses with receipts, then the income is taxable to you. Whether the IRS would consider it miscellaneous income or self-employment income is a tough judgment call, and it would be better to never have a 1099 issued in the first place. If you treat it as self-employment income, then you can subtract your expenses as business expenses which will zero out the income but which of course will require an upgraded version of TurboTax. If this is miscellaneous income, then you may not deduct your expenses against the income.
